1. Field of the Invention
The present invention is directed to a shielded compartment for a magnetic resonance apparatus for shielding electromagnetic radio-frequency fields.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for acquiring images of the inside of the body of an examination subject. In a magnetic resonance apparatus, rapidly switched gradient fields that are generated by a gradient system are superimposed on a static basic magnetic field that is generated by a basic field magnet system. The magnetic resonance apparatus also has a radio-frequency system that emits radio-frequency signals into the examination subject for triggering magnetic resonance signals and picks up the magnetic resonance signals that are triggered, from which magnetic resonance images are produced.
Magnetic resonance signals are electromagnetic signals in the radio-frequency range. Their exact wave range is dependent on the strength of the basic magnetic field. So that the reception, and thus the magnetic resonance images, remain as uninfluenced as possible by external radio-frequency disturbances, it is standard in magnetic resonance technology to arrange at least the actual basic field magnet of the basic field magnet system, a gradient coil system of the gradient system and antennas of the radio-frequency system in a shielded compartment. The shielded compartment forms a gap-free envelope for the components of the magnetic resonance apparatus with an attenuation, for example, of at least 90 dB for a frequency range from 10 through 200 MHz and is—apart from windows—formed of a steel plate or a copper foil. The shielded compartment, of course, also develops its shielding effect in the opposite direction from inside to outside. U.S. Pat. No. 4,651,099 discloses an embodiment of a shielded compartment. The shielded compartment is fashioned of ferromagnetic metal for an additional shielding of, for example, the stray field of the basic field magnet toward the outside.
Since, for example, a gradient amplifier of the gradient system that supplies the gradient coil system and a control system of the radio-frequency system connected to the antennas are arranged outside the shielded compartment, corresponding connecting lines, for example between the gradient coil system and the gradient amplifier, are conducted via filters integrated into the shielded compartment, so that radio-frequency disturbances cannot proceed thereover into the shielded compartment. U.S. Pat. No. 6,218,836 discloses an embodiment of said filter.
For generating gradient fields, corresponding currents are set in gradient coils of the gradient system. The amplitudes of the required currents thereby amount to up to several 100 A. The current rise and decay rates can be up to several 100 kA/s. Given an existing basic magnetic field on the order of magnitude of 1 T, Lorentz forces that lead to mechanical oscillations of the gradient coil system act on these time-variable currents in the gradient coils. These oscillations are transmitted via various propagation paths onto the surface of the magnetic resonance apparatus. There, the mechanical oscillations are converted into acoustic oscillations that ultimately lead to unwanted noise.
A number of passive and active noise-reduction measures have been disclosed for magnetic resonance apparatuses. The known passive noise-reduction measures at a magnetic resonance apparatus include, for example, attachment of noise-damping foamed materials in cladding parts toward the gradient coil system and/or an arrangement of flexible layers at and/or in the gradient coil system. U.S. Pat. No. 4,954,781, for example, describes such measures.
It is also known to line an installation room of the magnetic resonance apparatus with soundproofing or sound-damping materials of, for example, mineral wool in order to at least damp a noise emission from the installation room toward the outside.